This invention relates to a process for underground retorting of oil shale.
Researchers have now renewed their efforts to find alternative sources of energy and hydrocarbons in view of recent rapid increases in the price of crude oil and natural gas. Much research has been focused on recovering hydrocarbons from solid hydrocarbon-containing material such as oil shale, coal and tar sand by pyrolysis or upon gasification to convert the solid hydrocarbon-containing material into more readily usable gaseous and liquid hydrocarbons.
Vast natural deposits of oil shale found in the United States and elsewhere contain appreciable quantities of organic matter known as "kerogen" which decomposes upon pyrolysis or distillation to yield oil, gases and residual carbon. It has been estimated that an equivalent of 7 trillion barrels of oil are contained in oil shale deposits in the United States with almost sixty percent located in the rich Green River oil shale deposits of Colorado, Utah, and Wyoming. The remainder is contained in the leaner Devonian-Mississippian black shale deposits which underlie most of the eastern part of the United States.
As a result of dwindling supplies of petroleum and natural gas, extensive efforts have been directed to develop retorting processes which will economically produce shale oil on a commercial basis from these vast resources.
Generally, oil shale is a fine-grained sedimentary rock stratified in horizontal layers with a variable richness of kerogen content. Kerogen has limited solubility in ordinary solvents and therefore cannot be recovered by extraction. Upon heating oil shale to a sufficient temperature, the kerogen is thermally decomposed to liberate vapors, mist, and liquid droplets of shale oil and light hydrocarbon gases such as methane, ethane, ethene, propane and propene, as well as other products such as hydrogen, nitrogen, carbon dioxide, carbon monoxide, ammonia, steam and hydrogen sulfide. A carbon residue typically remains on the retorted shale.
In order to obtain high thermal efficiency in retorting, carbonate decomposition should be minimized. Carbonate decomposition consumes heat, lowers thermal efficiency and decreases the heating value of off gases. Colorado Mahogany zone oil shale contains several carbonate minerals which decompose at or near the usual temperature attained when retorting oil shale. Typically, a 28 gallon per ton oil shale will contain about 23% dolomite (a calcium/magnesium carbonate) and about 16% calcite (calcium carbonate), or about 780 pounds of mixed carbonate minerals per ton. Dolomite requires about 500 BTU per pound and calcite about 700 BTU per pound for decomposition, a requirement that would consume about 8% of the combustible matter of the shale if these minerals were allowed to decompose during retorting. Saline sodium carbonate minerals also occur in the Green River formation in certain areas and at certain stratigraphic zones.
Shale oil is not a naturally occurring product, but is formed by the pyrolysis of kerogen in the oil shale. Crude shale oil, sometimes referred to as "retort oil," is the liquid oil product recovered from the liberated effluent of an oil shale retort. Synthetic crude oil (syncrude) is the upgraded oil product resulting from the hydrogenation of crude shale oil.
The process of pyrolyzing the kerogen in oil shale, known as retorting, to form liberated hydrocarbons, can be done in surface retorts in aboveground vessels or in situ retorts underground. In situ retorts require less mining and handling than surface retorts.
In in situ retorts, a flame front is continuously passed downward through a bed of rubblized oil shale to liberate shale oil, off gases and residual water. There are two types of in situ retorts: true in situ retorts and modified in situ retorts. In true in situ retorts, the oil shale is explosively rubblized and then retorted. In modified in situ retorts, some of the oil shale is removed before explosive rubblization to create a cavity or void space in the retorting area. The cavity provides extra space for rubblized oil shale. The oil shale which has been removed is conveyed to the surface and retorted above ground.
While efforts are made to explosively rubblize the oil shale into uniform pieces, in reality the rubblized mass of oil shale contains voluminous different size fragments of oil shale ranging in size from boulders to minute fines. Smaller fragments of oil shale do not pack tightly and uniformly against the surface of large boulders. Furthermore, these different sized fragments create vertical, horizontal and irregular channels extending sporadically throughout the bed and along the wall of the retort. As a result during retorting, hot gases often flow down these channels and bypass large portions of the bed, leaving significant portions of the rubblized shale unretorted.
Different sized oil shale fragments, channeling and irregular packing and imperfect distribution of oil shale fragments cause many other deleterious effects including tilted (nonhorizontal), irregular, high temperature flame fronts in close proximity to the retorting zone and fingering, that is, flame front projections of high temperature which extend downward into the raw oil shale and advance far ahead of other portions of the flame front. High temperature flame fronts and fingering can cause carbonate decomposition, coking and thermal cracking of the liberated shale oil. Irregular, tilted flame fronts can lead to flame front breakthrough, incomplete retorting and burning of the product shale oil. Flame fronts in close proximity to the advancing front of the retorting zone can also cause combustion of the product shale oil. If a narrow portion of the flame front advances completely through the retorting zone, it can ignite the effluent oil and off gases and may cause explosions.
Oil shale boulder typically contains a large amount of oil which diffuses out very slowly over a long period of time. As the flame front of the combustion zone approaches the oil shale boulder, heated air often flows along the channel surrounding the boulder. Heated air in combination with the effluent oil from the boulder often ignites the oil. Extremely high temperatures will result and persist until the oil has stopped diffusing out of the boulder. Loss of oil is the result.
In the case of severe channeling, horizontal pathways may permit oxygen to flow underneath the raw unretorted shale. If this happens, all of the oil flowing downward in that zone may burn.
It has been estimated that losses from burning in in situ retorting are as high as 40% of the product shale oil.
Typifying the many methods of in situ retorting are those found in U.S. Pat. Nos. 1,913,395; 1,191,636; 2,481,051; 3,001,776; 3,586,377; 3,434,757; 3,661,423; 3,951,456; 4,007,963; 4,017,119; 4,126,180; 4,133,380; 4,149,752; 4,194,788 and 4,243,100. These prior art processes have met with varying degrees of success.
One particularly advantageous process of in situ retorting oil shale is described in U.S. patent application, Ser. No. 198,850, filed Oct. 20, 1980, by Gerald B. Hoekstra and David R. Christian, one half interest of which is owned by the assignee of the present invention. U.S. patent application, Ser. No. 198,850 is hereby expressly incorporated herein by reference.
It is, therefore, desirable to provide an improved process for in situ retorting of oil shale.